Re: [PATCH] btrfs: annotate lockless read of defrag_bytes in should_nocow()

[David Sterba]

On Wed, Apr 01, 2026 at 10:21:53AM +0800, Cen Zhang wrote:
> should_nocow() reads inode->defrag_bytes without holding inode->lock,
> while btrfs_set_delalloc_extent() and btrfs_clear_delalloc_extent()
> update it under that spinlock.
> 
> This is a data race.  The read is a quick check used to decide whether
> to fall back to COW for a NOCOW inode: if defrag_bytes is non-zero and
> the range is tagged EXTENT_DEFRAG, we force COW so that defragmentation
> can rewrite the extent.  Reading a stale value is harmless because:
> 
>   - A missed increment may skip COW once, but the defrag pass will
>     redo the extent later.
>   - A stale non-zero may force an unnecessary COW, which is a minor
>     efficiency loss, not a correctness issue.
> 
> On 64-bit platforms an aligned u64 load is naturally atomic so tearing
> cannot happen.  On 32-bit platforms u64 may tear, but we only test for
> zero vs non-zero, so the heuristic stays correct regardless.
> 
> Add READ_ONCE() to prevent the compiler from caching or splitting the
> load and to document the intentional lock-free pattern.

As it's explained it's ok to do the unlocked read but of that that the
dara_race() is more convenient, we do not really need the READ_ONCE
semantics.